A display included in a terminal equipment of a computer system receives video signals of various systems because different computers are different from each other, for example, in video signal scanning frequency, image displaying interval, displaying position and blanking interval, etc. Accordingly, a display capable of properly displaying images represented by video signals of different systems, i.e., what is called a multiscan display, has been used in recent years.
A display of such a type varies horizontal oscillation frequency according to the result of voltage conversion of the frequency of a synchronizing signal included in video signals provided to the display by a synchronizing oscillator as disclosed in Japanese Patent Publication No. 61-8628, originally published as Japanese Patent Application Laid-Open No. 53-57914, so as to deal with video signals of different scanning frequencies. Another display of such a type shifts the phase of a synchronizing signal included in video signals by a position correcting circuit using a monostable multivibrator as disclosed in Japanese Patent Appication Laid-Open No. 61-100078 so as to adjust the position of the image on the screen of the display according to image display position. There are many known techniques concerning multiscan displays.
The above-mentioned prior art employs a deflecting circuit comprising many active elements to deal with video signals of different systems and uses a circuit of a large scale to secure operating stability and reliability, causing problems in ease of manufacture and in adjustment of display. Recently, the scanning frequency of the video signals has been increased and hence the number of systems of video signals to be dealt with by the display has increased. As regards the scanning frequency of video signals, a display is required to be capable of dealing with video signals having horizontal scanning frequencies in the range of 15.75 kHz for NTSC video signals to about 100 kHz for high-definition video signals, and video signals used in CAD systems and CAM systems. A complex element constant switching control circuit is necessary to improve the prior art display so as to meet such a requirement, and a large-scale circuit is necessary to secure operating reliability. Furthermore, it is difficult to secure satisfactory performance to prevent the deterioration of picture quality and distortion of pictures for video signals of different systems, and the cost of the display increases to secure satisfactory performance.